Firefox problems

Now I like Internet Explorer. I have a Windows Logo on my Avatar, a Windows Logo on my Signiture, it is kind of obvious then that I designed the first few bits of my frontend in Internet Explorer.

However, however much I despise, hate, loath, want to kill Firefox I realise that in order to get people using the eventual software that will bare my frontend, I need it working in Firefox. Hence I have a problem, because I like what I have so far and I do not wish to compromise its design.

The problem is this, the description bar I have that shows when you hover over a SubTab Item on the Menu seems to want to stretch beyond the page when in Firefox. Not to dig at Firefox, which of course I am, but this Div is actually perfect and should work. Its only real point is to be 100% wide. Call me stupid, but I would have thought 100% wide means as wide as the page without the margin. It works for everything else, it works for the Menu blocks.

I was wondering if anybody had any ideas about the page and how to fix it. I have tried implicetly setting the width with JavaScript so that it is actually the width of the page minus the margin, but it does not work. I have tried specifically taking off increasing amounts of width after the initial page width, but ultimately found that it did not work. It looks perfect in Internet Explorer, it would be great to look perfect in Firefox

Disregard the bit beneath the menu, that has a serious problem with Firefox (doesn't everything) but I am on fixing that, I know what that is.